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Vectra A® - Liquid Crystal Polyester ( Vectra A )

Material Information

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Fiber Rod
Fiber Rod

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Common Brand Names:
Vectra A, Xydar
 
General Description:  

General Description : These are remarkable materials - not only are they high performance thermoplastics with the properties normally associated with that group, they also have very distinctive additional properties resulting from their liquid crystalline nature. They have stiff linear molecules which readily organise themselves spatially into zones with the three dimensional order typical of solid crystals. In general, this occurs both in solution and in the melt and it is strongly promoted by shear or flow resulting in a high degree of orientation in the flow direction. In the case of these melt-processable or thermotropic (strict meaning : forms liquid crystal regions in the melt) polyesters, it is in the melt that this property is of practical importance, but in the case of Kevlar, which decomposes below its "melting point", liquid crystals are formed in solution and are used to achieve its high degree of orientation and resultant properties.

One main consequence of this morphology is that the structure and properties of the material are highly anisotropic. The morpholgy is crudely similar to that of a fiber reinforced polymer but where the reinforcement is oriented molecules of the matrix material; they are often called self-reinforcing polymers (SRP) as a result. Design approaches based on the use of fiber reinforced polymers (FRP) can be applied, up to a point, to SRP's; it should also be noted that, in contrast to normal FRP's, the addition of fibers to SRP's (glass and carbon filled grades are relatively widely used) reduces rather than increases the degree of anisotropy. The morphology of SRP's can also can be compared to that of wood and the idea of grain direction can be useful in thinking about its nature.

The mechanical properties of Vectras are extremely high in the direction of flow/molecular orientation and correspondingly low in the other dimensions. Therefore, the properties of a melt-processed article depend very markedly on the details of the melt flow during its manufacture. The outer layers of an extruded or injection moulded item will normally be uniformly and highly oriented parallel to the surface; in the core, at least of straightforward injection mouldings, "tumbling" of the melt usually occurs leading to molecular orientation roughly perpendicular to the general flow direction. One consequence of this is that the common practice of machining prototype articles before making an injection moulding tool can lead to highly misleading results - the classic example is that of a flanged bobbin which, if machined from rod with its axis parallel to that of the rod, will have extremely weak flanges but when injection moulded will have strong, stiff flanges. Furthermore, the highly fibrillar nature of the outer layers of semi-fabricated items leads to other problems if they are machined eg fibrillation, tear-out, poor surface finish and potential dimensional tolerance difficulties. These two types of problem lead to a clear recommendation against machining from the polymer manufacturer and we ourselves have to disclaim liability for the results of any attempted machining; however, this leaves would-be prototypers in a nasty "no-win" situation to which we suspect that, short of making a "cheap" injection moulding tool, there is as yet no satisfactory solution. We therefore hope that these observations will help those of our end-users who see no option but to machine to do so on a basis of informed understanding.

With the exception of their very high impact strength which will be discussed in more detail later in this paragraph, published mechanical properties are broadly similar to those of glass fiber reinforced engineering polymers, but it follows from the previous paragraph that these properties depend to a much greater extent than usual on the details of the test specimen so they must be used with even more than normal caution. This is even more true of measurement of impact strength, particularly notched where results are dominated by whether/how the notch penetrates the highly oriented surface layers and it seems likely that conventional test results tend to underestimate impact strength. Nevertheless, it is clear even from these values that SRP's have very high impact strengths (not that much less than polycarbonate's) and very much greater than those of conventional fiber reinforced engineering polymers whose strength and moduli SRP's broadly match. Furthermore, a high proportion of this toughness is retained down to liquid nitrogen temperatures at least.

Vectra has good creep and fatigue properties but its wear characteristics, particularly for unfilled grades, are adversely affected by surface fibrillation. Its barrier properties are excellent (at least comparable to those of PVDC) and its radiation, UV and chemical and hydrolysis resistances are very good. It is insoluble in at least almost all solvents (and so very difficult to characterise scientifically) but attacked to some extent by strong acids and somewhat more by bases; however, even for bases a combination of elevated temperatures/long exposure times/quite high concentrations are needed before significant loss of properties occurs. Yet another of its remarkable properties is its low, though highly anisotropic, coefficient of thermal expansion (CTE) - to the extent that the CTE parallel to the flow direction has a (low) negative value. [This rare characteristic is shared by the highly oriented fibers - carbon, Kevlar and ultra high modulus polyethylene]. In practice, the CTE can be varied within limits by selecting grade and moulding conditions and components produced that match the CTE of glass, ceramic, some metals and glass fiber/epoxy laminates; this is used eg for surface mounted electronic components.

There are other consequences of SRP's morphology that favour injection moulding namely : low melt viscosity (once sheared), favouring long and complex melt flow paths; very low warpage and shrinkage, favouring production of high precision parts; very low heat of fusion, favouring fast cycle times. However, internal welds lines tend to be quite weak and it is most important that these are considered very carefully during design.

Applications appear to be in the fairly early stages of development but include mechanical and electronic components that require injection moulding to very close tolerances eg complex electronic connectors.

Colour : Its natural colour is an opaque brownish-yellow but it often coloured black.

   

Chemical Resistance

Acids - concentrated Good-Poor
Acids - dilute Good
Alcohols Good
Alkalis Good-Fair
Aromatic hydrocarbons Good
Greases and Oils Good
Halogenated Hydrocarbons Good(?)
Halogens Good
Ketones Good

Electrical Properties

Dielectric constant @1MHz 3.0
Dielectric strength ( kV mm-1 ) 47 @ 1.5mm
Dissipation factor @ 1MHz 0.02
Surface resistivity ( Ohm/sq ) 4x1013
Volume resistivity ( Ohmcm ) 1016

Mechanical Properties

Abrasive resistance - ASTM D1044 ( mg/1000 cycles ) 56
Coefficient of friction 0.12-0.14
Compressive strength ( MPa ) 70
Elongation at break ( % ) 3
Hardness - Rockwell M60
Izod impact strength ( J m-1 ) 520
Tensile modulus ( GPa ) 2-10
Tensile strength ( MPa ) 55-165

Physical Properties

Density ( g cm-3 ) 1.40
Flammability V0 @ 0.8mm
Limiting oxygen index ( % ) 35
Radiation resistance Good
Resistance to Ultra-violet Good
Water absorption - equilibrium ( % ) 0.03
Water absorption - over 24 hours ( % ) 0.02

Thermal Properties

Coefficient of thermal expansion ( x10-6 K-1 ) -5 to +75
Heat-deflection temperature - 0.45MPa ( C ) 220
Heat-deflection temperature - 1.8MPa ( C ) 180
Lower working temperature ( C ) ~ -200
Specific heat ( J K-1 kg-1 ) 1000
Thermal conductivity @23C ( W m-1 K-1 ) 0.18
Upper working temperature ( C ) 200-220

Properties for Vectra A® - Liquid Crystal Polyester Chopped Fiber

Property Value
Material Vectran HS
Coefficient of thermal expansion x10-6 K-1 -ve.
Density g cm-3 1.4
Extension to break % 3.3
Modulus GPa 65
Specific Modulus cN/tex 4800
Shrinkage @100C % =<0.5
Specific Tenacity cN/tex 210

Properties for Vectra A® - Liquid Crystal Polyester Fiber

Property Value
Material Vectran HS
Coefficient of thermal expansion x10-6 K-1 -ve.
Density g cm-3 1.4
Extension to break % 3.3
Modulus GPa 65
Specific Modulus cN/tex 4800
Shrinkage @100C % =<0.5
Specific Tenacity cN/tex 210

Properties for Vectra A® - Liquid Crystal Polyester Film

Property Value
Permeability to Oxygen @25C x10-13 cm3. cm cm-2 s-1 Pa-1 0.0002
Permeability to Water @38C x10-13 cm3. cm cm-2 s-1 Pa-1 400
All information and technical data are given as a guide only. Although every effort has been made to ensure that the information is correct, no warranty is given as to its completeness or accuracy.

Buy Vectra A® - Liquid Crystal Polyester on-line

We stock and supply the following standard forms:
Fiber Rod
Fiber Rod

Choose a form to search our on-line catalog  

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